1. Field of the Invention
The present invention relates to a recursive comb filter to be used for a video signal processing circuit of a magnetic recording and reproducing apparatus or the like.
2. Description of the Related Art
One of the techniques of elevating the picture quality of a reproduced video signal is a technique of removing noises with a recursive comb filter by utilizing the line correlativity in the vertical direction of a video signal.
In the NTSC television system, a luminance signal has a line correlativity and is substantially the same signal in an adjacent one horizontal scanning period (abbreviated as 1H hereinafter). A chroma signal is displaced in the phase by 180 degrees per line and therefore the present signal as inverted in the phase and the present signal as delayed by 1H are substantially the same signal. The noise component included in a video signal is usually so small in the level that a small amplitude part having no line correlativity is considered to be a noise component, a technique of removing noises by subtracting it from the original signal is adopted and a chroma noise reduction (CNR) circuit in which is used a recursive comb filter provided with also a color irregularity suppressing effect is used particularly on chroma signals.
That is to say, as shown in FIG. 4, the above mentioned CNR circuit comprises a subtracter 2, subtracter 3, limiter amplifier 4, switch 5, 1H delaying circuit 6 and inverting amplifier 7.
A chroma signal Sa having a 1H line correlativity is fed as an input to an input terminal 1. The input signal Sa becomes a signal Sd through the subtracter 2 and is fed to an output terminal 8. The signal Sd is delayed by 1H by the 1H delaying circuit 6, is inverted in the phase by 180 degrees by the inverting amplifier 7 and becomes substantially the same signal Se as the before 1H delay. The difference between the input signal Sa and 1H delayed signal Se is taken by the subtracter 3 and output as a signal Sb. A signal Sc obtained by amplifying the difference signal Sb by the limiter amplifier, and the signal Sc 4 and limiting the amplitude is input into the other input terminal of the subtracter 2 through the switch 5. In the subtracter 2, the signal Sc is subtracted from the input signal Sa and the subtraction signal Sd is output to an output terminal 8 as an output. The switch 5 is controlled by a non-correlation detecting signal to be off only when there is no line correlativity in the vertical direction. In a luminance signal system, when the difference between the present luminance signal and 1H delayed luminance signal is taken and the difference signal amount is above a predetermined value, it will be judged that there is no correlation before and after 1H and the non-correlation detecting signal will be made by a pulse making means.
The operation shall be explained in the following. In case a continuous chroma signal having a line correlativity has a noise component, the input chroma signal Sa containing the noise component will be subtracted from the signal Se delayed by 1H by the 1H delaying circuit 6 and a signal Sb having only the noise component extracted will be obtained. When the noise component, reduced k times (k&lt;1 wherein k is a gain) by the limiter amplifier, is subtracted from the input chroma signal Sa containing the noise component, a chroma signal Sd having the noise component removed will be output to an output terminal 7. In case a continuous chroma signal having a line correlativity has no noise component, Sa=Se and, as Sc=0, the output Sd will not be different from the input Sa.
Here, such step response that a chroma signal of an amplitude A is present until a horizontal line but is no longer present from the next line shall be considered. The gain k of the limiter amplifier 4 represents a recursive coefficient and is set to be 0&lt;k&lt;1.
First of all, the gain k of the limiter amplifier 4 is assumed to be k=0.5 and the operation in case the amplitude A of the input chroma signal Sa is below the limiter level L of the limiter amplifier 4 shall be explained with reference to FIG. 5. At this time, the switch 5 will remain on.
Sa represents a step-like chroma signal introduced into the input terminal 1 and Se represents a signal when the signal Sa is delayed by 1H by the 1H delaying circuit 6 through the subtracter 2 and is inverted in the phase by 180 degrees by the inverting amplifier 5. Sb represents a difference signal between the signals Se and Sa in the subtracter 3 and Sd represents a difference signal between the input signal Sa and a signal Sc when the signal Sb is suppressed by the limiter amplifier 4.
While the chroma signal Sa introduced into the input terminal 1 is of a fixed value A, Sc=0 and then, in the 1H period when the chroma signal is no longer present, Sa=0 and Se will still remain to be of the amplitude A and, on the basis of the output Sc=-kA of the limiter amplifier 4, the output Sd=kA will remain. Further, in the next 1H period, as Sa=0 and Se=kA, the output Sd=k2A and will be thus multiplied by k in turn to be attenuated. This is shown to tail the fluctuation suppression and to be effective to the noise suppression. That is to say, the limiter amplifier 4 is provided to limit the tailing amount.
The operation in case the level of the input chroma signal Sa is made to be more than twice as high as the limit level L of the limiter amplifier 4 and the gain k of the limiter amplifier 4 is made k=0.5 shall be explained in the following with reference to FIG. 6. The switch 5 shall remain on.
While the chroma signal Sa introduced into the input terminal 1 is of a fixed value A, Sc=0, in the 1H period when the color signal is no longer present, Sa=0, Se will still remain to be of the amplitude A, will be multiplied by k by the limiter amplifier 4 and will be suppressed to the L value of the limit level, the output signal Sc will become Sc=L and will be subtracted from the signal Sa and the output signal Sd=L will remain. In the next 1H period, Sd=kL and then Sd will be multiplied by k in turn to be attenuated.
Here, int he 1H period when the chroma signal is no longer present, in case a non-correlation detecting signal switches off the switch 5, Sc=0 will be able to be made, the output Sd=0 as shown by the dotted line in FIG. 6 and the tailing of the chroma will be able to be eliminated.
However, the problem is that it is very difficult to emit a non-correlation detecting signal most properly. In the case of controlling a chroma signal, it will be generally practiced to obtain a non-correlation detecting signal from a luminance signal but often it will not coincide with the variation of the chroma signal.
As described above, there has been a problem that the method of using a switch at the time of a non-correlation in order to reduce the tailing in case the variation of a large amplitude signal level is large requires a non-correlation detector for its control and will be properly operate in many cases.